Headphone and speaker protection circuit

The principle is relatively simple and will not be described in detail. From the circuit analysis, DW can be replaced by a resistor. The role of the Zener diode here is to use a relatively small delay capacitor to obtain a relatively long delay connection time, and the cut-off action is also more straightforward when the amplifier circuit has a DC output. The experimental results are indeed the same.

The input capacitance of the three-terminal voltage regulator is determined according to the load. If a high-power relay with a very small DC resistance is used, a capacitor of 470UF or more should be used. Since the resistance of this relay is relatively large, the actual measurement is about 1K, which means that the current consumption of this circuit should be below 20MA. In the experiment, a 47UF capacitor can work normally. It is feasible to use a 100UF capacitor in the circuit. If this capacitor is too large, it will not be possible to cut off the connection between the load and the amplifier when shutting down, causing an impact on the headphones. Since the working current of this circuit is very small, it is OK to replace the three-terminal voltage regulator circuit with 78M15 or 78L15.

Schematic diagram:

The basic functions are:
1. Delayed connection of headphones after power on. According to the board I made, the headphones are connected after a delay of about 3-5 seconds after power on to protect the headphones from the power on current shock.
2. Power off after power off. Since the filter capacitor of the power supply part is selected to be relatively small, after power off, the connection between the headphones and the amplifier is almost disconnected at the same time to protect the headphones from the power off current shock.
3. Output DC voltage abnormal protection. After a simple experiment, when the output voltage of +1.5V appears at the output end of the amplifier, the connection can be disconnected within 1 second, and when the amplifier has a negative voltage output, the protection action voltage is relatively high.

After 3 hours, I finally soldered the protection circuit of the headphone amplifier. Due to the lack of components, the parameters were different from the schematic diagram above. Fortunately, the soldering was successful in one go. Note that the small black rectangle between the input and output terminals is the Japanese miniature relay, which is most suitable for protecting headphones. Of course, you can also use a large relay and use it as a speaker protection. The PCB board has been made according to the installation form of dual relays.

Relays used in the circuit:

The finished circuit board is quite small, right? The area of ​​the PCB wrench is 10CM*4CM.